Aerosols consist of small particles, liquid or solid, that are suspended in air. Airborne dust particles, virus particles and bacteria can also comprise an aerosol.
The study of aerosols has significance in many areas such as environmental and health studies, Homeland Security, and military defense, especially when the aerosol particles are pathogens. The device for collecting aerosol particles, commonly known as an aerosol sampler, is a critical component for these studies; its function is to collect aerosol-containing air and to characterize the concentration and type of the aerosol particles contained in the air.
Aerosol particles entrained in moving air have mass and velocity vectors—i.e., they have momentum—as they are collected for sampling. Because of their mass, aerosol particles have inertia; they resist changes in their velocity vectors as their paths carry them into and through sampling collectors.
In a typical aerosol sampler, the aerosol-containing ambient air is first aspirated into the air sampler system through the air intake or inlet and thence transmitted to the rest of the sampler, sometimes through a system of transmission tubes or ducts, so as to convey the aerosol-containing air sample to the measurement portion of the aerosol-sampling device.
One of the basic performance parameters for an aerosol sampler is sampling efficiency. Overall sampling efficiency (η0) is the ratio of amount of aerosol material in the collected sample (Ccoll) to the amount of aerosol in the air (Cair) being sampled. It is usually calculated as the ratio of particles collected per volume of air sampled to the concentration (C) of particles in the ambient air.η0=Ccoll/Cair 
The goal of unbiased aerosol air sampling is to measure the actual amount of aerosol, measured as the number of aerosol particles or as the weight of the airborne aerosol material, in an air sample that is as representative as possible of the air being sampled.
Aerosol air sampling often takes place in windy settings, wherein the sampled air has to be pulled from horizontal flow into a vertically oriented air intake. Because of the inertial aspects of aerosol particles, the change in direction from horizontal to vertical can cause a portion of the massive aerosol particles to overshoot the air intake and, also, to impact against, and lodge upon, the walls of the air intake sampling tube.
Therefore, a goal of aerosol air sampling is to collect air samples in such a way that η0 is as close to 1 as possible, regardless of wind direction and speed or particle size. It may also be desirable for η0 to be greater than 1 if one wants to maximize the amount of aerosol collected. A high-efficiency air intake is one that is resistant to the influence of wind upon collecting efficiency. A high efficiency air intake is important for the gathering of reliable aerosol air sampling data.
Although air intakes or inlets for aerosol sampling devices can be very complex, in general, they are simple structures that typically consist of a pipe or stack that extends a distance upwards from the aerosol sampler to a debris/rain hat or cap/shell with an annular air intake opening. Many situations require that the opening of the inlet be distanced from the rest of the sampler enclosure to assure the aerosol sample is taken from an appropriate sampling height, or that it is taken from air that is not adversely affected by the flow around the enclosure or emissions from the sampler itself. This means that inlets for outdoor air sampling can range from several inches in height to over 6-feet above the sampler enclosure. As the inlet usually protrudes well above the aerosol sampler, it makes movement of the sampler awkward; therefore, the inlet is usually detachable and removed. In sampling in hazardous environments, the sampling crews may risk prolonged and unhealthful exposure if such crews need to attach and/or detach inlets for aerosol sampling.